This book studies and analyzes an important phenomenon occurs downstream multi-vent hydraulic structures, which called reverse flow. During maintenance programs, some vents may be closed and others are kept open. If one or more side vents are opened (or closed) and the vents on the other side are closed (or opened), it is a case of unsymmetric operation. The main flow body is separated into two main jets as reversal and forward flows. The area of reverse flow has a negative effect on flow characteristics downstream hydraulic structures.An extensive experimental program has been conducted on a five vents regulator model.The velocity vectors downstream the piers were measured by making a mesh of points to investigate the zone of reverse flow, and the length of separation. Statistical model was developed, to predict the percentage area of reverse flow, when side gates are operated. The predicted model is in a good agreement with the experimental results. For application point of view, the optimum side-gates operation can be decided during maintenance programs.